A New Form of Matter: II

NASA-supported researchers have discovered a weird
new phase of matter called fermionic condensates.

February 12, 2004: We learned it in grade school. There
are three forms of matter: solids, liquids and gases.

But that's not even half right. There are at least six: solids, liquids,
gases, plasmas, Bose-Einstein condensates, and a new form of matter
called "fermionic condensates" just discovered by NASA-supported
researchers.

"This
is a very exciting time," says University of Colorado/NIST physicist
Deborah Jin, lead scientist for the group who produced the first fermionic
condensate in Dec. 2003. "My group works extremely hard these
days. Both the excitement of a major advance and the competition to
be first have been driving forces."

News of their landmark achievement appeared in the Jan. 24-30 online
edition of Physical Review Letters.

Most second graders can recite the properties of ordinary solids,
liquids, and gases. Solids resist deformation. They're stiff and they
can crumble. Liquids flow, they're hard to compress, and they assume
the shape of their container. Gases are less dense, they're easy to
compress, and they not only assume the shape of their container ...
they expand to completely fill it.

The fourth form of matter, the plasma, is gas-like, made of atoms
that have been ripped apart into ions and electrons. The sun is made
of plasma, as is most of the matter in the universe. Plasmas are usually
very hot, and you can keep them in magnetic bottles.

The fifth form, the Bose-Einstein condensate (BEC), discovered in 1995,
appears when scientists refrigerate particles called bosons to very
low temperatures. Cold bosons merge to form a single super-particle
that's more like a wave than an ordinary speck of matter. BECs are fragile,
and light travels very slowly through them. (Read Science@NASA's "A
New Form of Matter" to learn more about BECs.)

Now we have fermionic condensates--so new that most of their basic
properties are unknown. Certainly they're cold. Jin created the substance
by cooling a cloud of 500,000 potassium-40 atoms to less than a millionth
of a degree above absolute zero. And they probably flow without viscosity.
Beyond that...? Researchers are still learning.

"When you find a new form of matter," notes Jin, "it
takes a while to understand it."

Fermionic condensates are related to BECs. Both are made of atoms
that coalesce at low temperatures to form a single object. In a BEC,
the atoms are bosons. In a fermionic condensate the atoms are fermions.

What's the difference?

Bosons
are sociable; they like to get together. As a rule of thumb, any atom
with an even number of electrons + protons + neutrons is a boson.
So, e.g., ordinary sodium atoms are bosons, and they can
merge to become Bose-Einstein condensates.

Fermions, on the other hand, are antisocial. They are forbidden
(by the "Pauli Exclusion Principle" of quantum mechanics)
to gather together in the same quantum state. Any atom with an odd
number of electrons + protons + neutrons, like potassium-40, is a
fermion.

Jin's group found a way around the antisocial behavior of fermions.
They used a carefully applied magnetic field to act like a fine-tunable
"Cupid." The field causes loner atoms to pair up, and the strength
of that pairing can be controlled by adjusting the magnetic field.
Weakly paired potassium atoms retain some of their fermionic character,
but they also behave a bit like bosons. A pair of fermions can merge
with another pair--and another and another--eventually forming a fermionic
condensate.

Jin suspects that the subtle pairing of atoms in a fermionic condensate
is the same pairing phenomenon seen in liquefied helium-3, a superfluid.
Superfluids flow without viscosity, so fermionic condensates should
do the same.

A closely related phenomenon is superconductivity. In a superconductor,
paired electrons (electrons are fermions) can flow with zero resistance.
There is intense commercial interest in superconductors because they
could be used to produce cheaper, cleaner electricity, and to build
high-tech marvels like levitating trains and ultra-fast computers.
Unfortunately, superconductors are difficult to handle and study.

Fermionic condensates might help.

The biggest problem with today's superconductors is that -135°C
is the warmest temperature at which any of them can operate. The liquid
nitrogen or other cryogenics needed to cool the wires down make any
apparatus using superconductors expensive and bulky. Engineers would
rather work with superconductors at room temperatures.

"The strength of pairing in our fermionic condensate, adjusted for
mass and density, would correspond to a room-temperature superconductor,"
notes Jin. "This makes me optimistic that the fundamental physics
we learn through fermionic condensates will help others design more
practical superconducting materials."

Left:
Pairs of fermions can get together and act like bosons. In this diagram,
the spins of paired particles are aligned. In Jin's work they are
opposite.

NASA has many uses for superconductors. For instance, gyros that
keep satellites oriented could use frictionless bearings made from
superconducting magnets, improving their precision. Also, because
superconductors can carry the same amount of current as copper in
a much smaller wire, the superconducting electric motors aboard spacecraft
could be 4 to 6 times smaller than ordinary motors, saving precious
volume and weight.

Others speculate that superconductors could play a role in a permanent
Moon base, such as the one in President Bush's recently announced
vision for future human exploration of space. Superconductors would
be a natural choice for ultra-efficient power generation and transmission,
because ambient temperatures plummet to -173 °C during the long
lunar night. And during the months-long journey to Mars, a "table
top" MRI machine made possible by superconducting wire would be a
powerful diagnosis tool to help ensure the health of the crew.

Editor's
note: This story mentions six phases of matter: solids, liquids,
gases, plasmas, BECs and fermionic condensates. Physicists debate
the total. You could add to the list many other forms such as liquid
crystals, glasses, ferromagnets, paramagnets and so on. Do fermionic
condensates rightly belong among the major categories, like liquids
and solids, or with less fundamental subdivisions such as liquid crystals?
This question will be answered in the months and years ahead as researchers
learn more about the properties of coalescing fermions.

more information

NASA's Office
of Biological and Physical Research (OBPR) supports studies of
fundamental physics for the benefit of people on Earth and in space.
Jin has received funding for her work from the OBPR, the National
Science Foundation, NIST, and the Hertz Foundation.

Why potassium?
But of all the fermionic atoms (viz., those made from an odd number
of particles), what's so special about potassium? The choice isn't
as arbitrary as it may seem: it comes as a consequence of the cooling
methods used by Jin's group.

The first step in cooling a vapor to such extremely low temperatures
is a technique called "laser cooling", which brings the gas to within
a few tenths of a degree above absolute zero (~0.1 Kelvin). Then the
atoms are placed in a magnetic trap, and the highest-energy atoms
are allow to escape -- like sweat evaporating from your skin -- thus
cooling the remaining atoms to within 100 billionths of a degree (100
nK). The final step, which is a new addition since Jin's landmark
work in 1999, is to transfer the atoms to an optical trap and continue
the evaporative cooling, eventually reaching the critical temperature
where the vapor should condense into a superfluid, about 50 nK.

The laser-cooling step works best on elements from the first column
of the periodic table. Of those seven elements, only lithium-6 and
potassium-40 are stable, long-lived fermions. All of the research
groups working in this area are using one of these two elements, Jin
says. (The stable bosons from this column -- sodium and rubidium --
are used for making BEC's.)